Electron transfer processes are key steps of vital biological phenomena such as DNA synthesis and aerobic respiration. Electron transfer events take place in proteins between distant prosthetic groups such as iron- sulfur clusters, heme units, flavins and copper centers. The latter comprise the blue copper and CuA units, designed for performing efficient long range electron transfer. This efficiency seems to be highly related to the electronic structure of these metal sites. Blue copper centers possess mononuclear rigid structures able to accommodate either Cu(II) or Cu(I) ions. The CuA site is a mixed-valence bimetallic unit displaying an unusual electronic structure. The broad goal of the parent grant is to define the structural factors that control electron transfer rates in Ru-labeled copper and iron proteins. The broad objective of the present project is to uncover the structural features responsible for the ET properties of blue copper and CuA centers in proteins by means of paramagnetic NMR spectroscopic studies of native and mutated copper proteins in which different metal ligands have been replaced. Paramagnetic NMR is able to identify the metal ligands and to provide the electron spin density onto each nuclei belonging to the coordinated residues. The electron transfer performance in the mutant proteins will be analyzed at the light of these studies.